1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a power semiconductor device of high breakdown voltage provided with a diode.
2. Description of the Background Art
An example of the power semiconductor device may include a power module of high breakdown voltage that can withstand a voltage, for example, greater than or equal to 600V. Such a power module may be equipped with an IGBT and a diode.
For example, Japanese Patent Laying-Open No. 2009-283781 discloses a semiconductor device provided with a diode, in which an n-type semiconductor substrate has an anode formed on one main surface and a cathode formed on the other main surface. The anode is a p-type diffusion region and the cathode is formed of an n-type super-high concentration impurity layer and an n-type high concentration impurity layer. A guard ring is formed so as to surround this anode. A p-type diffusion region on the cathode side is formed in the region of the cathode facing the guard ring.
When a voltage in the forward direction is applied between the anode electrode and the cathode electrode of the above-described semiconductor device, the diode is brought into an ON state. In this state, a plurality of carriers are accumulated within the semiconductor substrate (drift layer). In other words, holes are injected from a p-type diffusion region into a drift layer, while electrons are injected from an n-type high concentration impurity layer or the like into the drift layer. In contrast, when a voltage in the reverse direction is applied between the anode electrode and the cathode electrode, the diode is brought into an OFF state. In this state, the electrons and the holes in the carriers accumulated in the drift layer are discharged from the cathode electrode and the anode electrode, respectively.
In this OFF state, since a p-type diffusion region is formed on the cathode side, the volume of the n-type region on the cathode side is decreased. This allows reduction of the current concentration in the outer peripheral end of the anode in the OFF state. In other words, the current concentration is reduced in the region in the vicinity of the guard ring in the OFF state, which causes an increase in the breakdown voltage in this region.
However, in the semiconductor device disclosed in Japanese Patent Laying-Open No. 2009-283781, the current in the forward direction may decrease which flows through the main region in the diode (drift layer sandwiched between the anode and the cathode) in the ON state. This causes a decrease in the density of carriers accumulated in the drift layer in the ON state, which may result in a decrease in the amount of the current that should essentially flow into the diode.
The present invention has been made in view of the above-described problems. An object of the present invention is to provide a semiconductor device which prevents rise and drop of the voltage in the forward direction flowing through the diode and allows an increase in the breakdown voltage in the vicinity of the edge termination region such as a guard ring during the recovery operation in which the diode is switched from the ON state to the OFF state.